The PDF can be computed from the definition and is pretty straight forward, the CDF however has no close form and can only be approximated.I've developed a function for the PDF and a set of functions using different approximations for the CDF.You can read my analysis in:http://formaldev.blogspot.com.au/2012/09/T-SQL-NORMDIST-1.htmland either copy paste the ones you want from the posts or get them from the project page for the blog posts at:https://tsqlnormdist.codeplex.com/

My advice:INDEXing a poor-performing query is like putting sugar on cat food. Yeah, it probably tastes better but are you sure you want to eat it?The path of least resistance can be a slippery slope. Take care that fixing your fixes of fixes doesn't snowball and end up costing you more than fixing the root cause would have in the first place.